1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading an image using an image sensor.
2. Related Background Art
A demand has arisen for developing a high-resolution, compact, color image reading apparatus which reads image data from a document carrying the image data. As a technique for meeting such a demand, as proposed in U.S. patent application Ser. Nos. 061,047 (filed on June 12, 1987), 724,300 (filed on April 7, 1985), and 544,240 (filed on Oct. 21, 1983) by the present applicant, a solid-state scan technique is known. More specifically, an apparatus using a combination of a photodiode array and a MOS switch, or using a semiconductor function element having both a pixel separation function and an optical data storage function, is known. Such a solid-state imaging element is highly miniaturized. Therefore, the size of one chip is much smaller than that of a document. For this reason, a reduction optical system must be used between a document surface and the element.
On the other hand, when an optical system is used which has a one-to-one focusing correspondence, such as a focusing optical transmission array in which a large number of focusing optical fibers are aligned in arrays, an apparatus does not become large in size unlike the case wherein the reduction optical system is used. However, in consideration of color balance or assurance of gray scale levels, a linear light receiving element array must be used over the total width of a document.
For this reason, in recent years, studies on development of a contact sensor have been made. The present applicant proposed an image reading apparatus using the contact sensor in U.S. patent application Ser. Nos. 023,968 (filed on May 10, 1987), 704,920 (filed on Feb. 25, 1985), and 804,110 (filed on Dec. 3, 1985). In an arrangement using the contact sensor, since the reduction optical system need not be used, the apparatus can be rendered compact. However, it is difficult to prepare an elongate sensor capable of covering the total width of a document. Therefore, a plurality of line sensors are arranged in a line or in a staggered manner.
Therefore, signals separately read by a plurality of lines of line sensors must be linked to obtain a continuous signal for a line. In this case, if the characteristics of the line sensors or the characteristics of processing circuits corresponding to the line sensors are not uniform, a good image output cannot be obtained.